Rocky Mountain spotted fever, caused by Rickettsia rickettsii, is an important infection in the United States owing to the extreme severity of untreated cases, the difficulty in establishing an early clinical or laboratory diagnosis, and the lack of an effective means of prevention. Transmitted by tick bite, the organisms are spread by the bloodstream throughout the body, entering and injuring endothelial cells of the microcirculation. The in vitro model of Rickettsia rickettsii endothelial cell interaction utilized in Dr. Silverman's laboratory has led during the current funding cycle to highly productive research with establishment of important new concepts of infectious disease pathogenesis. The proposed study will examine cultured human endothelial cells infected with the virulent Sheila Smith strain and the avirulent Iowa strain of R. rickettsii and the virulent Breinl and avirulent Madrid E strains of R. prowazekii. Current evidence from Dr. Silverman's laboratory strongly suggests that R. rickettsii induced endothelial cell injury is mediated by reactive oxygen species. Superoxide radical has been detected in large amounts in culture supernatants during internalization of rickettsiae into endothelial cells. The investigator postulates that injury is initiated as a result of aborted phagocytosis of the organism that results in an oxidative burst. The oxidative burst releases superoxide at the cell surface which is converted to toxic H202 by extracellular superoxide dismutase in the endothelial glycocalyx. H202 readily passes through the plasma membrane and is capable of directly causing peroxidation of membrane lipids, or it may react with additional superoxide to produce the even more toxic hydroxyl radical. Catalase and pyruvate are known scavengers of H202 and protect endothelial cells from hydrogen peroxide generated from superoxide. The hypothesis that R. rickettsii causes abortive phagocytosis leading to endothelial cell injury following pretreatment with these scavengers, measuring intracellular glutathione and comparing them to comparable experiments with the Iowa, Breinl and Madrid E strains.